/usr/lib/mono/4.5/OpenSystem.C.dll is in libmono-opensystem-c4.0-cil 3.2.8+dfsg-4ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/4.5/../gac/OpenSystem.C/4.0.0.0__b77a5c561934e089/OpenSystem.C.dll